1. Field of the Invention
The present invention describes a process for preparing alpha-beta-unsaturated carboxylic acids by dehydration of hydroxycarboxylic or alkoxycarboxylic acids which are substituted in the alpha position, in particular 2-hydroxyisobutyric acid (2-HIBA), while avoiding accumulation of by-products and additives.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
Such processes are adequately known from the prior art. CH 430691 describes the dehydration of 2-HIBA dissolved in methanol in the liquid phase using NaOH as catalyst to form MMA and MAS. The catalyst is added in only small amounts. To achieve the high temperature of 260° C., phthalic anhydride and tetraethylene glycol dimethyl ether are used as bath liquid.
DE1568948 discloses a dehydration of 2-HIBA dissolved in alcohol or molten 2-HIBA in the liquid phase in the presence of catalysts consisting of basic compounds which lead to formation of hydroxyisobutyrate anions and in the presence of high-boiling, polar solvents.
In a manner similar to the above, DE1211153 discloses a dehydration of 2-HIBA dissolved in alcohol or of methyl 2-hydroxyisobutyrate (M-2-HIB) in the gas phase over various fixed-bed catalysts at 250-400° C. Catalysts used are, inter alia, supported phosphates, supported sulphates, Al2O3, ZnO, MoO3—Al2O3, MgO, BPO4.
Catalysts disclosed in DE1768253 are alkali metal and alkaline earth metal salts of 2-HIBA (Na, K, Li, Ca, Mg, Ba, Sr) used as, for example, hydroxides, carbonates, sulphites, acetates or phosphates. The dehydration is preferably carried out at atmospheric pressure and 210-225° C. with addition of polymerization inhibitors. This disclosure also describes continuous introduction of the catalyst and partial discharge of the reactor contents in order to avoid accumulation of catalyst and by-products here. However, recovery of the target product which is necessarily likewise discharged is not described.
EP 487853 discloses a process for preparing methacrylic acid (MAA), which comprises the following steps: a) preparation of acetocyanohydrin (ACH) from acetone and HCN, b) synthesis of hydroxyisobutyramide (HIBAm) by hydrolysis of ACH over MnO2, c) homogeneous catalytic conversion of HIBAm into MHIB using methyl formate or MeOH/CO with formation of formamide and d) hydrolysis of MHIB to 2-HIBA with subsequent dehydration to form MAA and H2O. The last reaction step is described as continuous with introduction of stabilizers. The difficulties which inevitably result therefrom in long-term operation by accumulation of by-products, etc., are not discussed.
According to DE 1191367 the dehydration of alpha-hydroxy carboxylic acids is carried out in the presence of Cu and hydroquinone as polymerization inhibitor and also a mixture of alkali metal chlorides or bromides and corresponding halide salts of Zn, Sn, Fe, Pb as catalyst at temperatures of 185-195° C. Continuous operation and possible problems associated with recirculations are not described. The inventors' own experiments show that, firstly, the use of halide salts as catalysts result in formation of alpha-halogenated reaction by-products which have to be separated off again in a complicated manner from the actual target product, and secondly the use of halogenated compounds requires the use of appropriately resistant technical materials, which make the process more expensive, because of their corrosive action.
According to DE 102005023975 the dehydration is carried out in the presence of at least one metal salt, for example alkali metal and/or alkaline earth metal salts, at temperatures of 160-300° C., particularly preferably from 200 to 240° C. Metal salts mentioned there as being suitable include, inter alia, sodium hydroxide, potassium hydroxide, calcium hydroxide, barium hydroxide, magnesium hydroxide, sodium sulphite, sodium carbonate, potassium carbonate, strontium carbonate, magnesium carbonate, sodium bicarbonate, sodium acetate, potassium acetate and sodium dihydrogen phosphate. A particular aspect is that the pressure of the dehydration stage is the same as that in a preceding transesterification stage and is preferably in the range 0.1-1 bar. Discharge of by-products is not disclosed.
It is common to these processes described in the prior art, in particular those configured as a continuous variant, that polymerization inhibitors have to be added to the reactor feed or the reaction mixture introduced into the distillation column which generally follows the reactor in order to avoid polymerization of the target products. Without further measures for removal or discharging, the substances continually accumulate in the reactor, associated with corresponding disadvantages such as deposit formation or discolouration.
Furthermore, the reaction of the alpha-substituted carboxylic acids is never complete. The recirculation of unreacted reactants is therefore absolutely necessary in order to achieve an economical industrial process. The documents cited in the prior art discuss neither such a necessary product recirculation nor the formation and elimination of by-products. In addition, regardless of the catalyst used in the reaction of, for example, alpha-hydroxy carboxylic acids, by-products typical for this reaction are always formed at least in traces even in the case of very good selectivities of >98%. All these by-products have a boiling point similar to that of the alpha-substituted carboxylic acids and thus inevitably accumulate in the recirculation of the unreacted starting materials. The same applies to polymerization inhibitors which are added to the reaction and have boiling points similar to that of the starting material.
Thus, if no further technical measures are undertaken, the secondary components or additives described accumulate continuously in the recirculation of starting material, which firstly leads to an overall increase in the recycle stream and increases the energy consumption in the respective revaporization in the reactor circuit. To avoid this continuous accumulation of undesirable by-products, a discharge measure by means of which a constant by-product or additive level can be realized in the reaction system is therefore necessary. However, from an economic point of view undifferentiated discharge is disadvantageous since target product and catalyst are also lost alongside the undesirable by-products and excess polymerization inhibitors.
Although the publication “Avoiding Accumulation of Trace Components” (Ind. Eng. Chem. Res. 1992, 31, 1502-1509) discusses, in numerous variants, the possible discharge of by-products which occur in traces, there is no discussion of recirculation of any concomitantly discharged target products downstream of the discharge point.
It is therefore an object of the present invention to avoid accumulation of undesirable by-products and additives, in particular excess polymerization inhibitors, by discharging these from the reaction process in order to achieve a stable concentration equilibrium and at the same time to minimize the loss of target product and to optimize the energy consumption for maintaining the circulation streams. Further objects which are not explicitly mentioned can be derived from the overall contents of the following description, claims and examples.